The subject matter disclosed herein generally relates to pistons for internal combustion engines and more particularly to pistons with reduced top land height.
Internal combustion engines typically include a piston disposed inside a cylinder that may be provided with a cylinder liner. Piston rings that fit into a groove on the outer diameter of the piston are typically provided. The main task of the piston is to convert thermal energy into mechanical work. The piston may include a piston head, a top land, a pin support, and a skirt. Piston rings seal the combustion chamber from the crankcase and distribute and control the oil. The piston rings also stabilize the piston.
In the operation of internal combustion engines, it is common to see a decrease in power resulting from unburned fuel trapped within the top land height by the clearance formed between the top land and the cylinder liner (or the cylinder bore where there is no cylinder liner). The term crevice volume may be used herein for the purpose of describing this clearance volume where unburned fuel is trapped. This unburned fuel reduces engine efficiency and increases total hydrocarbon emissions.
One approach to reduce the top land crevice volume is the implementation of a reduced top land height. However, reduced top land heights increase top ring groove temperatures and cause ring groove deposits.
In the operation of internal combustion engines, it is common to see a decrease in power resulting from the accumulation of deposits in the combustion chambers. These deposits result from the burning of fuel and oil. The deposits are primarily composed of carbon.
The buildup of carbon deposits in the top land of a piston causes wear (polishing) of cylinder liners and carbon raking (vertical lines formed on the liner wall caused by carbon being raked down the liner). Such wear may result in the need to replace the cylinder liners. The wear may also result in increased maintenance and spare parts costs. Carbon buildup also affects performance of the internal combustion engine, including a reduction in output and efficiency and an increase in oil consumption.
One approach for dealing with carbon deposits is the implementation of a Tight Top Land (TTL) profile. In a TTL profile, the clearance between the top land and the cylinder liner is reduced. TTL profiles control the fundamental factors which drive deposits, namely temperature and residence time. Piston temperatures are reduced with a TTL profile because heat conduction out of the piston to the cylinder bore has been enabled and because the heat flux into the top land has been reduced. With reduced temperatures, deposits are less likely to form. Another reason for the cleanliness of the TTL profile is because the oil which is on the top land and in the top ring groove is being constantly replenished with a fresh supply of oil between the land and the liner due to the tight clearances. With this replenishment, the residence time of the oil on the top land and in the top ring groove is reduced, and with reduced residence time, deposits are also reduced.